Piezoresistive microcantilevers have been widely used in micro-electro-mechanical systems (MEMS) sensor arrays for the detection of chemical vapors and gases, where the embedded signal transducer (the piezoresistor) provides a more compact, rugged, and low-power alternative to the optical feedback mechanisms inherited from atomic force microscopy.
It is well known in the art, particularly with respect to chromatographic analysis of gas flows, that a gas or simple mixture of gases (e.g., a binary mixture, a ternary mixture, etc.) can be identified by measuring the gas' thermal conductivity. This is typically achieved by placing a resistance temperature detector (“RTD”), a thermocouple, etc., in the gas to be identified. In order to measure the electrical resistance of the RTD, a small amount of power is supplied which causes the RTD temperature to be greater than the gas temperature. As a result of the temperature difference between the RTD and the surrounding gas, the electrical power delivered to the RTD is dissipated into the gas as heat. If the gas composition changes (for example, goes from 100% nitrogen to 95% nitrogen/5% argon), the thermal conductivity of the gas also changes. The result is a change in heat dissipation efficiency of the RTD, with an increase/decrease of heat dissipation leading to a decrease/increase, respectively, in RTD temperature, under constant power conditions. The prior art teaches a typical strategy, in which the electrical power is adjusted in a feedback mechanism to maintain a constant RTD temperature. Alternatively, the power can be kept constant, and the RTD temperature allowed to fluctuate freely with changes in gas composition. Since the RTD has a large temperature coefficient of resistance (e.g., approximately 4000 ppm/K, for a platinum resistance thermometer), the change in temperature with gas composition leads to a change in resistance that is readily measured by a Wheatstone bridge circuit.
In typical thermal conductivity sensor designs, the sensor power is continuously varied to keep the sensor temperature constant, requiring an active temperature feedback control mechanism. This design has some limitations, particularly regarding response time to thermal conductivity changes.
A similar apparatus disclosed in the prior art is used to determine gas flow rate. As will be described below, the RTD temperature varies with the following gas properties: 1) thermal conductivity, 2) temperature, and 3) flow rate. A Wheatstone bridge configuration may be used with several RTDs arranged such that the thermal effects are substantially eliminated, and only variations of flow rate affect the RTD temperature.
In other prior art, a mechanical vibration of a rigid body in a fluid (gas or liquid) has been exploited as a diagnostic and scientific tool. The vibrating body (“oscillator”), typically driven to bulk oscillation by a piezoelectric element, possesses intrinsic resonant frequencies that are altered upon interaction with certain external influences. This operating concept is prototypically embodied in the quartz crystal microbalance (QCM), where the addition of mass to the oscillator causes a shift in the resonant frequencies that are detected. Typically, the fundamental or a low harmonic frequency is monitored. Such a device is well known in the art of thin film deposition, where they are often referred to as thickness monitors. Furthermore, gas and chemical vapor detection is enabled with the addition of coatings to which the gas-phase species have some tendency to bind. This approach has since been widely applied to QCM devices for sensing applications. Devices based on the propagation of surface acoustic waves (SAW), as opposed to the bulk acoustic waves that exist in a QCM, have also become widely investigated as sensors. The use of vibrating coated microcantilevers for chemical vapor detection exists in the prior art.
However, the selection of detector coatings for gases at typical temperature and pressure conditions of interest (e.g., ambient, atmospheric) is often problematic. First, many gases under such conditions have little thermodynamic tendency to partition into the bulk of common sensing materials (e.g., polymers). This is the result of low cohesive energy densities and hence small solubility parameters, particularly for non-polar gases. Second, since chemisorptive interactions are usually exploited as a means of detecting and identifying gas-phase analytes, highly inert species, such as N2 and noble gases, are virtually undetectable by sensors using chemically functionalized materials (e.g., alkanethiol self-assembled monolayers). In the absence of such coatings, the sorption tendencies of the oscillator surface are low (i.e., zero/near-zero sticking coefficient) at or near ambient atmospheric pressures due to the persistent saturation coverage of up to a few mono-layers of adsorbed gases such as oxygen and carbon monoxide. Therefore, the mechanism of frequency shift due to mass addition to the oscillator is inappropriate for non-reactive gases such as N2 and noble gases.
None of the teachings heretofore available in the prior art provide a method of selective gas detection which allows: 1) that gases are detected at concentrations of interest (e.g., parts-per-million); and 2) that gases are mutually distinguished from one another, whether they occur individually or simultaneously in a mixture. Gas sensors that are based on the heat conduction process described above suffer from non-unique responses, since many pure gases (or mixtures thereof) may have the same thermal conductivity. Similarly, gas sensors based on resonant frequency shifts are limited by the nonuniqueness of viscosity and density amongst possible gas analytes. Therefore, a gas sensor which is capable of overcoming the different problems associated with sensors using the dissimilar physical mechanisms of gas sensing described above would aid greatly in selective gas detection.